Telephone signals are provided to subscribers through the public switched telephone network ("PSTN" or "the network"). The subscriber portion of the network has two wires known as "tip" and "ring". These wires carry information being transferred to and from the subscribers, as well as control signals, such as a connection request (or "ring") signal. The bandwidth of the network is between about 300 Hz to 3.4 KHz. Accordingly, any data terminal equipment (DTE), such as data modems, facsimile machines, (non-cellular) portable telephones, speaker phones, and message answering machines, for example, must be compatible with the network (PSTN) to function properly. To this end, data access arrangements (DAAs) provide an interface to bridge any inconsistencies between the data terminal equipment (DTE) and the network (PSTN).
Furthermore, the network (PSTN) must be protected from damage due to, for example, faulty data terminal equipment (DTE) or inadvertent shorts through the data terminal equipment (DTE) to its power line. Indeed, the United States Federal Communications Commission ("FCC") requires a 1500 volt isolation between the data terminal equipment (DTE) and the public switched telephone network (PSTN). In the past, data access arrangements (DAAs) used transformers to provide such electrical isolation. Although transformers adequately isolated the network from the DTE and although transformers permitted bi-directional signal transfer (i.e., an AC signal on a primary would induce a signal on the secondary and an AC signal on the secondary would induce a signal on the primary), they have several limitations. First, transformers are costly relative to solid state devices. Second, transformers are relatively large and heavy. Thus, transformers are not well-suited for applications requiring the interface to have minimal volume and weight, e.g., portable DTEs such as portable personal computers, portable facsimile machines, and portable modems. Therefore, an inexpensive, small, and lightweight data access arrangement (DAA) is needed.
Moreover, the data terminal equipment (DTE) are typically four wire devices, having separate transmit and receive wire pairs. Accordingly, the data access arrangements (DAAs) must include a duplexing circuit, or transhybrid, to bridge the two-wire network and the four-wire data terminal equipment (DTE). Since data can be transmitted and received simultaneously, the transhybrid must separate the transmit and receive signal paths. This separation is achieved by suppressing the level of the transmit signal at the output of the transhybrid, and inverting this signal to form a transmit cancellation signal. This signal is added to the receive input of the transhybrid, thereby separating the transmitted signal from the received signal. In known DAAs, the transmit cancellation signal is derived from the output of the line drive circuit. Unfortunately, the cancellation signal cannot be independently controlled. Thus, a DAA having an improved transmit-receiver separation circuit is needed.
The data access arrangement (DAA) should ideally have a flat frequency response, a constant group delay, extremely small amplitude and frequency distortion, and should match the impedance of the network line.